Field of the Invention
The present invention relates to the field of Lithium ion batteries.
Related Art
State of the art Li-ion negative electrodes employ graphitic active materials with theoretical capacities of 372 mAh/g. Development of new high-capacity anode materials, such as Sn, Si and other alloy anodes, has been one of the major focuses of the research in lithium-ion battery field.
Silicon (Si) possesses a theoretical capacity of 4200 mAh/g, while Tin (Sn) has a theoretical capacity of 994 mAh/g for full lithiation to the Li22M5 phase wherein M is a metal, such as Si or Sn. However, despite their remarkable high capacity and the intensive research done in the field, there have been no widespread applications of Si or Sn alloy anodes in lithium-ion cells, mostly due to the large volume change associate with lithiation and delithiation of the material. This volume change disrupts the integrity of electrode and induces excessive side reactions, leading to fast capacity fade. Several polymer binders were successfully applied for the alloy anodes, such as carboxymethyl cellulose (CMC) and polyacrylic acid (PAA). It has been shown that a thin oxide layer with a thickness of several nanometers exist in the commercial Si particles. The interaction between silanol and the carboxylic acid groups (see FIG. 1b) on CMC or PAA plays a very important role to maintain the adhesion of binder on the active material, as well as to keep the mechanical and electronic integrity of the electrode. However, the Si-based cell performance from the current binders continues to be unsatisfactory in terms of long-term capacity retention and efficiency. There is a demand to develop polymer binders with new structural moiety which offers better binding strength, which helps to avoid detachment of the binder from active particles during volume shrinkage after delithiation.
Since Lee et al. identified the catecholic amino acid 3,4-dihydroxy-L-phenylalanine (DOPA) as the main protein component that offers strong interfacial adhesion in mussel, polymer adhesives containing a catechol moiety (see FIG. 1a) have attracted major attention in this field. Since it is well-established that an oxide layer exists on the surface of the commercial metal alloy anodes, it is proposed that a polymer binder (see FIG. 1c) with a catechol side chain could interact with this surface oxide layer, generate a strong adhesion strength and maintain the electrode integrity during drastic volume change in lithiation and delithiation.